The slow inhibitory postsynaptic potential (slow IPSP) was recorded from the 9th or 10th paravertebral sympathetic ganglia of bullfrogs by the sucrose-gap technique. Adrenergic alpha antagonists, phentolamine and dehydroergotamine, up to concentrations of 10 and 100 uM, respectively, did not significantly reduce the amplitude of the slow IPSP; however, when concentrations of phentolamine greater than 10 uM were used, the slow IPSP as well as other synaptic potentials were all nonselectively depressed. Propranolol, a beta adrenergic antagonist, in a concentration of 1 and 10 uM also did not affect slow IPSP amplitude significantly; higher concentrations of this compound reduced the slow IPSP and other synaptic potentials non-selectively. U-0521, a COMT inhibitor, in a 50 ug/ml concentration, did not show any potentiating effect on the size of the slow IPSP. These results do not provide support for an adrenergic mediation of the slow IPSP, but they are consistent with the proposal that the PSP is a muscarinic cholinergic response.